Witnesses told cops 45-year-old Markus Leo Husarek was doing some martial arts moves when he allegedly knocked the victim down at Jefferson and Mason streets at 4:40 p.m.

As the victim walked passed him, the apparently deranged Husarek “grabbed him around the neck from behind and threw him to the ground,” police said.

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The two men were complete strangers, police said.

The elderly man, a San Francisco resident, was transported to California Pacific Medical Center with life threatening injuries. He suffered lacerations and required staples on his head, police spokesman Officer Albie Esparza said. His condition has since improved.

“He is at home resting,” Esparza said Tuesday.

Disgusted witnesses flagged down cops when they witnessed the attack, which occurred at a usually bustling intersection filled with tourists.

The cops arrested Husarek without further incident.

He was booked into county jail on a felony charge of assault with a deadly weapon, since in this case his “karate fist” is considered a weapon, Esparza said.